A continuing objective in the design of gun propellants is to provide a gun propellant which is energetic when deliberately ignited, but which exhibits high resistance to accidental ignition from heat, flame, impact, friction, and chemical action. Propellants possessing such resistance to accidental ignition are known as "low vulnerability ammunition" (LOVA) gun propellants.
Conventional LOVA gun propellants comprise an elastomeric binder, throughout which are dispersed particulates of high-energy material, particularly oxidizers. The elastomeric binder is generally a cured elastomer, formed, for example, by the urethane reaction of a multi-functional prepolymer with a multifunctional isocyanate. Examples of such LOVA gun propellants are described, for example, in U.S. Pat. Nos. 4,263,070 and 4,456,493, the teachings of which are incorporated herein by reference. Generally, LOVA propellant grains are formed by extrusion at elevated temperatures whereat substantial curing takes place. Because the grains cure to some extent as they are being formed, control of extrusion conditions is difficult. If cured LOVA propellant is unused, it cannot be recycled, and burning the propellant is generally the only suitable disposal method.
Another type of LOVA propellant has a binder of cellulose acetate or a cellulose acetate derivative. An example of this type of propellant is described in U.S. Pat. No. 4,570,540, the teachings of which are incorporated herein by reference. These types of LOVA propellants are solvent processed, a process which entails relatively long processing times and a large number of steps. Also, the use of solvent creates environmental problems.
The present invention is directed to LOVA propellants which use thermoplastic elastomers as binders. Thermoplastic elastomers have been previously used in propellants for rocket motors or the like, for example, as described in U.S. Pat. No. 4,361,526 and U.S. patent application Ser. No. 06/925,660 filed Oct. 29, 1986, the teachings of each being incorporated herein by reference. Gun propellants, however, are considered to be a different art than rocket motor propellants. Rocket motor propellants typically contain a particulate metal fuel, e.g., particulate aluminum. Gun propellants, on the other hand, should be substantially free of any metal, and for that matter, should be generally free of any material which leaves a solid residue in the barrel of the gun upon burning. Gun propellants should also be substantially free of chlorine, which degrades the gun barrel.
Furthermore, rocket motor grains are typically formed in a different manner. Gun propellant grains typically take their shape from the extrusion process and must be sufficiently solid when leaving the extruder to retain their extruded shape. Material for rocket motor propellants may be extruded, but generally large rocket motors assume their shape from a mold, e.g., the rocket motor case; thus, after leaving an extruder or mixer, a propellant composition for a rocket motor should be free-flowing or at least moldable so as to be able to assume the shape of the large mold.